Study of local children finds link between mercury in mothers, ADHD

ANIKA CLARK

Wednesday

Oct 10, 2012 at 12:06 AMOct 10, 2012 at 6:10 PM

SouthCoast babies born to mothers with higher mercury levels later showed signs of attention deficit hyperactivity disorder more often than those whose mothers had lower levels, according to a new study out of Brigham and Women's Hospital.

SouthCoast babies born to mothers with higher mercury levels later showed signs of attention deficit hyperactivity disorder more often than those whose mothers had lower levels, according to a new study out of Brigham and Women's Hospital.

The study also suggested what might seem a counter-intuitive link: Babies of mothers who ate a lot of fish during pregnancy less commonly showed ADHD-like behavior.

The findings from babies born at New Bedford's St. Luke's Hospital in the 1990s don't prove cause-and-effect relationships. And by itself, the study isn't sufficient to be the basis of a policy decision, according to senior author Dr. Susan Korrick, an assistant professor medicine at Brigham and Women's and Harvard Medical School.

But "in general, it's fair to say from this study that it's probably a healthy thing to eat fish during pregnancy, but you must avoid fish with high mercury," she said. "I say that, in part, because there are a lot of other studies that have basically demonstrated the same thing."

Almost all fish and shellfish contain trace amounts of mercury, according to the Food and Drug Administration. But larger fish that have lived longer contain more, the FDA says, and sharks, swordfish, king mackerel and tilefish pose the highest risk.

An FDA fact sheet updated last year recommends pregnant women avoid eating these fish but says they can eat up to 12 ounces weekly of lower-mercury fish and shellfish, such as salmon and shrimp.

The local study started with 788 babies whom researchers recruited at birth between 1993 and 1998. All were born at St. Luke's Hospital to mothers living in the city, Acushnet, Fairhaven and Dartmouth. Researchers determined mercury levels using hair from the mothers taken days after they gave birth. They also gave women questionnaires shortly after delivery asking how often they ate different kinds of fish. Researchers followed up with 607 of the babies when the children were seven to 10 years old.

Researchers had hair mercury data for 421 of their mothers and fish consumption data for 515.

"For moms whose hair mercury was greater than one part per million during pregnancy, increasing mercury above that level was associated with about a 60 percent increased risk of ADHD-related behavior," said Korrick. She said hair from 66 of the 421 moms had this mercury threshold and, as their mercury levels increased, so did the incidence of ADHD-related behavior in their children.

The study identified this behavior using different techniques, including a rating scale asking teachers questions such as how much trouble a child has sitting still in school and how often he or she loses assignments. Another was a computerized test measuring children's attention and impulse control. Researchers also used a standardized IQ test, according to Korrick, who said that children with clinical ADHD tend to perform poorly on certain parts of these tests. Although researchers charted associations using the other testing mechanisms, Korrick said the apparent protective effects of fish were most consistently seen via the teachers' reports as were the negative effects of mercury.

Though teacher reports are more subjective than the other tools, "there's no way, in theory, that the teachers would know which kids' moms had high hair mercury when they were pregnant," she said. And less precise measures tend to make it tougher to spot an association, not easier, she said.

The study was originally designed to examine whether early life exposures to PCBs affected children's development due to concerns about the polluted New Bedford Harbor, according to Korrick. The researchers previously identified relatively low PCB exposures in the children but they still noted an association with ADHD-related behaviors. Researchers are also studying if any exposure of these babies — who are now teenagers — to lead, manganese and arsenic is linked with other cognitive and behavioral measures. The latest report was published online in the Archives of Pediatrics & Adolescent Medicine.

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